Cellular polymers such as, for example, polyethylene, polystyrene, or polyurethane are of value in many application areas including notably thermal insulation. In this case it is particularly advantageous that such foam exhibits attractive dimensional stability and a relatively stable thermal insulation performance. Both of these trails are largely determined by the cellular structure of the foam and by the composition of the gas(es) within the cell cavity. A finer cell structure generally confers better insulation properties to a foam. However, as cells become finer, that is of small diameter, the compressive strength and dimensional stability of the foam frequently becomes inferior. Susceptibility to poor dimensional stability is greater when the cell cavity contains a gas which is likely to condense or alternatively diffuse out of the cell. Either phenomenon leads to a loss of partial gas pressure within the cell, resulting in poor dimensional stability of the foam. Exemplary of a cell gas noted to diffuse out of a cell cavity, especially of polyurethane, is carbon dioxide. The use of carbon dioxide to prepare cellular polymers and especially polyurethane is presently highly favored as a substitute for many of the commonly used physical blowing agents considered harmful to the ozone layer.
In support of this trend there exists a need to develop an improved process for the preparation of cellular polymers which exhibit attractive dimensional stability and thermal insulation properties. Particularly for polyurethane foam, there exists a need for a process which permits the use of, for example, water-generated carbon dioxide and yet provides for a foam having a commercially acceptable dimensional stability and thermal insulation performance.
From the general understanding of foam morphology and the formation of fine celled polymers it is to be appreciated that as the average cell size decreases the closed cell content of the foam becomes significant. A cell is comprised of a plurality of struts and corners defining a plurality of window regions. By "closed cells" it is generally understood that the window regions are completely sealed by a membrane thereby providing an isolated cell cavity. A high closed cell content frequently results in poor dimensional stability of the foam.
A potential solution to the dimensional stability problem is to provide a foam with an open-celled structure. That is a structure where one or more of the cell windows are completely open and not sealed by a membrane. Such open cells can not suffer from loss of partial pressure and ensuing poor dimensional stability. Expanded polymer having an open celled structure may be prepared by crushing the expanded polymer after its preparation to break cell windows. The latter means is only available for elastomeric polymers since crushing of rigid cellular polymers would lead to permanent deformation and damage. To provide for cell opening in a rigid polymer use of a cell opening agent during the process of its manufacture is required. Typically such cell opening agents are high boiling liquids such as for example high molecular weight poly(oxyalkylene) adducts. However, a disadvantage associated with the use of such cell opening agents is that they generally promote the formation of polymer containing a coarse cell structure and consequently unattractive thermal insulation properties.
Thus there exists a need to provide a foam having an "open-cell trait" regarding dimensionally stability but which retains a fine cell structure. To this purpose, the concept of preparing foam with a perforated cell window has been investigated. By "perforated" it is to be understood that the cell windows retain a membrane contacting the struts and corners, but that such membrane has voids within it, that is "pin-holes" or perforations. Thus there exists a need to discover substances which are capable of providing a perforated cell membrane and yet which do not particularly provide an open-cell structure. From a theoretical consideration, it is necessary to discover substances which are capable of promoting a localized perturbation leading to "pin-hole" formation within the cell membrane. Again from a theoretical consideration, substances mostly likely to promote a localized perturbation are those substances which have little or no interaction, physical or chemical, with the cellular polymer. Substances noted for having little or no interaction with their environment include for example poly(tetrafluoroethylene), or "PTFE".
EP-224,945-A discloses a polyurethane foam prepared in the presence of from 25 to 60 parts by weight per 100 parts of polyol of particulate PTFE having a particle diameter of 25 microns or less. U.S. Pat. No. 3,753,933 discloses a polyurethane foaming process in the presence of as little as 9 parts/100 parts polyol of a larger sized PTFE particle. However it is to be noted that use of large sized particles or high loadings in a polyurethane foaming process are not easily processed due to viscosity constraints or and provide foam with an undesirably course, open-celled structure.